Astigmatism is a defect in the eye that is corrected by a lens with a non-spherical prescription. The prescription, which is usually expressed as cylinder on the patient's prescription order, causes at least a portion of the surface of the lens to have the shape of a toric segment. Hence, such lenses are called toric lenses. The corrective lens must be properly oriented with respect to the eye of the wearer. That is, the intended top of the lens must be at the top of the wearer's eye. For ordinary glasses this presents no problem, because the lens is permanently fixed to the frame at the correct rotational orientation. The ear and nose pieces of the frame assure that the frame and the lens do not rotate with respect to the wearer's eyes. For contact lenses orientation is subject to constant alteration. In the case of contact lenses whose function is to correct astigmatism this is unacceptable.
Soft contact lenses which have been designed for use to correct astigmatism are well known in the art. Generally these lenses rely on some type of ballasting method to cause the lens to ride at the proper location on the eye. An ideal lens for correcting astigmatism has three properties:
(1) Good rotational orientation. That is, as stated above, the intended top of the lens should be located at the top of the wearer's eye when the lens is worn. A small amount of deviation from the correct orientation can be tolerated, provided the lens fitter measures the deviation and takes it into account in the lens's prescription.
(2) Good rotational stability. That is, the lens should remain at a fixed rotational orientation within the eye during the entire wearing period. Furthermore, the lens should assume the same orientation each time it is worn.
(3) Comfort. In general the thinner the lens and the smoother the lens surface, the more comfort it will provide.
Lenses in accordance with this invention are preferably soft contact lenses, which may be formed of many materials including hydroxyethylmethacrylate, metallo-organic substances, silicone rubbers and various other materials such as described in U.S. Pat. Nos. 3,503,942 and 3,639,524. The preferred soft contact lenses are hydrophilic; that is, they absorb water and, in fact, the water becomes an integral part of their structure. Hydrophilic contact lenses made in accordance with U.S. Pat. No. 4,405,773 are especially preferred for practicing this invention.
U.S. Pat. No. 4,084,890, discloses a contact lens which is alleged to have improved adherence to the cornea and improved visual acuity as a result of its configuration. The lens tapers from a thin outer edge to an intermediate point of maximum thickness and then tapers to the center of the lens. The peripheral surface portion is wedge shaped in cross section. However this lens is believed to be uncomfortable because of its thickness.
U.S. Pat. No. 4,095,878, discloses a contact lens which maintains the correct orientation when placed on the eye as a result of a flattened portion on the periphery. In one embodiment the lens is symmetrical about the horizontal axis. The lens is thinnest at its vertical axis and increases in thickness along the flattened section in both directions from the vertical axis, having its thickest portion at the horizontal axis. While the disclosed lens has rotational stability, the lens of the present invention is expected to have better rotational stability.
U.S. Pat. No. 4,324,461, discloses a contact lens for correcting astigmatism which is alleged to provide the proper rotational positioning. The lens has a thickness disparity between the superior (top) and inferior (bottom) portions of the lens and a thicker ballast portion is provided at the inferior portion. However the thicker ballast portion of the lens is believed to cause discomfort compared to the lens of the present invention.
While the posterior surface of a contact lens is generally spherical in configuration, where the lens is to used to correct astigmatism it will have a toric configuration. That is, the curved portion of the posterior of the lens has a major and minor axis; the radius of curvature of the posterior surface of the lens being longer in the major-axis direction than in the minor-axis direction. The result is that rather than being of a spherical configuration, the posterior of the lens has a toric configuration with the major axis running orthogonal to the minor axis. The major diameter of the toric curve is generally smaller in diameter than the overall lens, and is cut into a starting base curve which has a spherical configuration.
In the toric lenses of the prior art the junction between the base curve of the posterior surface of the lens and the toric curve portion is in an area of sharp inflection resulting in a sometimes uncomfortable fit to the cornea.